Failure avoidance effective silt fence

ABSTRACT

A silt fence structure and method of controlling erosion are disclosed. The silt fence structure includes a water permeable fence, an apron having first and second edges, the first edge providing a buried apron toe, the second edge attaching to the water permeable fence, and at least one flow barrier attached to the water permeable fence and to the apron so as to impede lateral runoff flow along the water permeable fence.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is claims benefit of U.S. Provisional PatentApplication No. 60/852,102 entitled “FAILURE AVOIDANCE SILT FENCE,”filed Oct. 16, 2006, the contents of which are hereby incorporated byreference.

FIELD OF THE INVENTION

This disclosure relates to soil retention methods and devices ingeneral, and, more specifically, to a failure avoidance silt fence.

BACKGROUND OF THE INVENTION

Construction and large-scale landscaping operations often involve theremoval or displacement of topsoil and plant matter serving to preventerosion. Silt, clay, sediment, and other ground matter may then be sweptaway by runoff. The silt and other material may be carried suspended inrunoff water until it is deposited in creeks, rivers, or still bodies ofwater. This can have a deleterious effect on wildlife and theenvironment. Traditional silt fences have been used as a measure forcontrolling runoff and erosion and to slow the displacement of silt andits subsequent deposition in undesirable locations. A traditional siltfence may be nothing more than a semi-permeable barrier placed in theflow of runoff.

A number of problems arise with conventional silt fences. These include,among others, failure of the silt fence to trap sediment due to scourfrom concentrated flow along the toe, resulting in undercutting of thetoe and discharge of sediment underneath the fence. Failure to trapsediment due to failure of the posts and excessive stretching andsagging of the fence can allow sediment to flow over the fence. Thisphenomenon is known as overtopping of the fence. Failure to trap clayand fine silt due to inadequate settling time has also been a problem.

Attempts to solve the problem of post failure and excessive sagging havebeen focused on using stronger posts and adding a wire mesh backing tothe fence. These solutions are problematic because of the added costsfor materials and installation.

One solution for the failure of a traditional silt fence to trap clayand fine silt due to inadequate settling time was to provide an ox-bowshaped installation formed by extending the downstream end of a siltfence uphill. This creates an impoundment at the downstream end. Thissolution is also problematic since impoundment drainage areas aretypically sufficiently large that a significant quantity of runoff waterand sediment is concentrated at the point. As a result, the impoundmentis likely to fill up, resulting in uncontrolled sediment flowovertopping the silt fence or flowing around the uphill end.

What is needed is a system and method for addressing the above, andrelated, problems.

SUMMARY OF THE INVENTION

The present invention disclosed and claimed herein, in one aspectthereof, includes a silt fence structure. The silt fence structure has awater permeable fence, and an apron having first and second edges, thefirst edge providing a buried apron toe, the second edge attaching tothe water permeable fence. At least one flow barrier is attached to thewater permeable fence and to the apron so as to impede lateral runoffflow along the water permeable fence. The water permeable fence may alsoprovide a buried fence toe.

In one embodiment, the at least one flow barrier has a generally righttriangular shape providing first and second edges attaching to the waterpermeable fence and the apron, respectively. In another embodiment, theat least one flow barrier has a curved triangular shape with first andsecond straight legs attaching to the water permeable fence and theapron, respectively, and a curved leg spanning between the waterpermeable fence and the apron. A plurality of flow barriers may attachto the water permeable fence at substantially the same location.

In some embodiments, the silt fence structure will comprise at least onefence post attached to the water permeable fence. A backing plate may bepositioned along the fence post and attached to the fence post such thatthe water permeable fence and the flow barrier interpose the backingplate and the fence post where the flow barrier is attached to thefence.

In some embodiments, a floculant is applied to the apron so as todisperse into runoff over the apron. In another embodiment, at least onegeo textile pouch containing floculant is attached to the apron so as todisperse floculant into runoff over the apron. In another embodiment,the apron is further comprised of first and second layers with afloculant placed therebetween so as to disperse into runoff over theapron.

The present invention, disclosed and claimed herein, in another aspectthereof comprises a method of controlling erosion on a sloped soilsurface having an upstream and a downstream direction. The methodincludes planting a plurality of fence posts into the soil, attaching asilt fence between the plurality of posts, and providing an apron alongthe fence near the toe portion extending overground from the fence in anupstream direction to an apron toe. The apron toe may be buried in thesoil. At least one flow barrier may be attached between the fence andthe apron.

In one embodiment, the method includes spraying a floculant onto theapron. In another embodiment, the method includes imbuing the apron witha floculant.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and is not meantto be limited by the accompanying drawings, in which like referencenumbers indicate similar parts:

FIG. 1 is a perspective view of one embodiment of a silt fence accordingto aspects of the present disclosure;

FIG. 2 is a perspective view of another embodiment of a silt fenceaccording to aspects of the present disclosure;

FIG. 3 is a comparison chart illustrating the efficiency of atraditional silt fence versus a silt fence according to aspects of thepresent disclosure;

FIG. 4 is a perspective view of another embodiment of a silt fenceaccording to aspects of the present disclosure;

FIG. 5 is a perspective view of a flow barrier attached to an apron andfence according to aspects of the present disclosure;

FIG. 6 is an exploded view of a flow barrier, fence, and fence postattachment according to aspects of the present disclosure; and

FIG. 7 is a close up view of a fence post attached to a silt fenceaccording to aspects of the present disclosure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a perspective view of one embodiment of a silt fence 100according to aspects of the present disclosure. Also illustrated in FIG.1 are the contour lines C of the prevailing landscape along with thearrows F indicating the direction of flow of runoff. In someembodiments, an apron 102 is provided to protect the soil 103 at a toe104 of a fence wall 108. Flow barriers 106 may be provided to preventconcentrated runoff flow along the toe 104. Application ofpolyacrylamide or another floculant, as described in greater detailbelow, may be used to aggregate fine particles and enhance settling. Thecombination of the strength of the fence wall 108 and spacing D of theposts 110 keeps stretching and sagging to an acceptable level underanticipated water and sediment loading. The apron 102, fence wall 108,and flow barriers 106 may be made from various geotextile materials. Thematerials chosen may be selected based upon the desired strength,longevity, and overall cost of the fence 100.

Some embodiments described herein will use a different material for theapron 102 than the fence wall 108. The apron 102 may be constructed witha heavier and tightly woven geotextile material to control the flow ofwater beneath the apron 102. The heavier material will minimize thestretching of the fabric during mechanized installation and spread verywell on the ground. This may also minimize seepage of the impounded flowthrough the apron 102.

In one embodiment, the materials of the fence wall 108 and apron 102 areattached by sewing the pieces together with at least one single seamusing polyester thread. Hot glue may be applied continuously, or atintervals, on the stitch to prevent any unraveling of the thread. Someembodiments will use hot glue exclusively.

In operation, as runoff approaches the fence 100 in the direction of thearrows F, it flows over the apron 102, which protects the toe 104 of thefence wall 108. An uphill end, or toe 112, of the apron 102 may beanchored by laying it in a toe trench and backfilling it. The toe trenchmay be compacted and the apron 102 is folded back over the compacted toetrench protecting the buried apron toe 112 and preventing erosion. Thus,in one embodiment, the problem of toe scour and undercutting seen in theconventional silt fence design is addressed by locating an apron upslopeof the fence wall 108.

Settling time can be increased by the introduction of the flow barriers106 along the fence 100 and application of a floculant such aspolyacrylamide. Polyacrylamide (PAM) aids in flocculation (aggregation)of fine particles and decreases the settling time, when mixed with therunoff flow. This enhances the trapping of clay and fine silt particles.Examples of application of polyacrylamide within the context of thepresent disclosure discussed further below.

A series of impoundments 114 (shown containing runoff) may be formed bythe interception of the flow along the fence by flow barriers 106. Thismay prevent concentrated flow from forming along the toe 104 of thefence 100 and increase the detention time allowing more sediment tosettle from the runoff. Additionally, the series of small impoundmentsresult in settling sediment more or less continuously along the fenceand may prevent the overtopping problem posed by forming an ox-bow shapeat the downstream end.

With respect to sagging and post failure, the silt fence wall material108 of the present disclosure may be made of a fabric that is strongerthan conventional fence with posts 110 appropriately spaced so thatunacceptable stretching does not occur. The final selection of fabricand post spacing may be based on computer and/or laboratory testing todetermine the maximum post spacing for each fabric before unacceptabledeformation occurs, along with an economic analysis of the trade-offbetween installing fewer posts with stronger (more costly) fabric orinstalling more posts and using lower-strength fabric. In oneembodiment, stronger fabric will be constructed by reinforcing thepolypropylene woven fabric (conventional silt fence material) withpolyester fibers. This technique may mitigate the need to install a wirebacking in addition to the fence itself and the cost associatedtherewith. A series of small impoundments 114 defined by the structureof the fence 100 may also aid in uniform load distribution along thefence 100 preventing excessive stretching and sagging at low spots.Runoff is shown in the impoundments 114 for illustration.

Referring now to FIG. 2, a perspective view of another embodiment of asilt fence 200 according to aspects of the present disclosure is shown.The silt fence of FIG. 2 is similar to the silt fence of FIG. 1 withsome modification as described below. In order to further minimize thedisturbance caused to the parent soil during installation, the toetrench installation method can be eliminated. Instead, the toe 112 ofthe apron 102 is tucked into a narrow slit in the ground and compacted.A vibratory plow or high speed concrete cutting saw may be used to makethe narrow slit in the ground wide enough to insert the silt fencematerial. To prevent the wind from blowing underneath the apron duringheavy storms, the toe 104 of the fence wall 108 may be buried along witha downhill apron toe 201.

In the present embodiment, the shape of the flow barriers 202 is changedfrom straight triangular (as in FIG. 1) to a curved triangular structureto facilitate trapping. The curved shape functions as a bag of sorts andintercepts and stores sediment. Additionally, to accommodate the changein flow contours (topography) along the construction site, the flowbarriers 202 may be installed as dual barriers as shown in FIG. 2. Thiswill enhance the trapping of the sediment as the runoff flow will beintercepted by either of the flow barriers at each post.

The silt fence 200 of FIG. 2 is illustrated as being installed along twodistinct slopes as illustrated by the placement of the contour lines Cand the arrows F indicating runoff direction. However, it is understoodthat the fence 200 could be installed along a single slope. Furthermore,any of the embodiments described herein may be useful for installationalong single slopes or multiple slopes on the prevailing landscape asdictated by the needs of the end user.

As described, a floculant may be used with various embodiments of thesilt fences disclosed herein to aid aggregation of fine soil particles.Anionic polyacrylamide (PAM) is one possible choice of floculant. PAM issoil specific and needs to be mixed well with the runoff flow foreffective flocculation. Anionic PAM is available is several physicalforms with various formulations. In one embodiment, liquid emulsionanionic PAM will be sprayed onto the material of the fence wall 108and/or apron 102 and dried. As the runoff flow approaches the fence PAMis mixed at the impounding areas 114 and flocculates fine particles.

In another embodiment, the apron 102 may comprise at least two layers(not shown). Anionic PAM in crystal form may be spread on a non-porouslayer of the apron 102 and then covered with a layer of porouspolypropylene woven material. The bottom apron material and the poroustop woven material are sealed at the edges to form a bag structure andhold the PAM crystals. As the runoff water approaches the fence, waterseeps through the porous woven material and mixes with the PAM crystals.Thus, PAM is released at the impounding area for aggregation of fineparticles. In yet another embodiment, anionic PAM wafers are stuffed ina meshed geo textile tube (not shown) and laid along the apron toe. Asthe runoff flow approaches the fence, it passes through the meshed geotextile tube and mixes with the PAM wafers. Thus, with any of thesemethods, PAM is released into the flow and flocculation takes place inthe impounded area.

Referring now to FIG. 3, a comparison chart illustrating the efficiencyof a traditional silt fence versus a silt fence according to aspects ofthe present disclosure is shown. A silt fence constructed according theprinciples described above (without the use of PAM) was evaluated at thesilt fence testing site (SFTS) at USDA ARS Hydraulics Lab. The fence wasevaluated under a wide range of testing conditions. Tests were performedwith the aid of an artificial rainfall simulator specifically designedfor the silt fence testing site. In every test it was observed that thescouring of the toe and excessive stretching of the fence weresubstantially eliminated. Detention time and trapping of sedimentdrastically increased due to addition of the flow barriers.

FIG. 3 illustrates the performance of a silt fence (denoted “FAEST”)built as described herein when compared to a conventional silt fence.Conventional silt fence performance data was obtained from the same siltfence testing site. Results of six field tests with identical testingparameters were considered here. The test parameters covered acombination of three different soil textures and two slopes along thefence. The ratio of total sediment load from the source area plot (EP)to total sediment discharged with the flow along the fence (UF) wascalculated for a side by side comparison. For clarity, the ratios arepresented as percentages in FIG. 3.

A value of 100% or greater would indicate that there was more sedimentdischarged with the flow along the fence than there was sedimentgenerated from the source area. In a conventional silt fence design,this would imply that additional sediment was generated by scouring ofthe toe trench which would lead to the failures described above. With asilt fence designed as described in this disclosure, the runoff flowfrom the source area would have to first fill the impoundments before itstarts flowing around the barriers and flows along the apron toe. Thus,the total sediment load discharged in the flow along the fence is lessthan the sediment load discharged from the source area. These behaviorscan be observed with the field test results depicted in FIG. 3.

It can be seen in FIG. 3 that with a conventional silt fence, 5 out of 6tests resulted in more sediment load discharge than what was generatedfrom the source area. This increase resulted from erosion along thefence. Also, conventional silt fence test results indicated that thefence failed with 2 out of 3 soils at 13% slope along the fence andsediment trapping efficiency was very poor.

Results of the field tests for the fence designed according to aspectsof the present disclosure indicated that the new design eliminated thefailure modes of a conventional silt fence and performed superiorly intrapping sediment. With 4 out of 6 comparison tests the ratio of totalsediment load at the source area to sediment load upstream of the fenceturned out to be much less than 1, indicating significant trapping ofsediment. The overall trapping efficiency of the designed fence as shownin the present disclosure was estimated to be around 90%. Also, due toincreased detention time behind the fence, the trapping of fine silt andclay particles increased when compared to a conventional silt fence.Testing under extreme conditions also demonstrated that the flow acrossthe apron does not disturb the soil under the apron and that fabricinstalled with a vibratory plow stays anchored.

FIG. 4 is a perspective view of another embodiment of a silt fence 400according to aspects of the present disclosure. In this embodiment, theshape of the flow barriers 402 is changed to a straight triangular shapeand is attached such that it is always perpendicular to the fence wall108. This embodiment can result in lowered construction costs whileretaining substantially all of the benefits described in the presentdisclosure. A computer simulation model of the hydraulics and sedimenttrapping was developed and used to simulate the impact of physical andhydrologic variables that determine the efficiency of the fence. Theseshowed that the high trapping efficiency of the previously describeddesigns may still be achieved with the modified flow barriers 402 ofFIG. 4.

The flow barrier 402 may be constructed out of the same material as thefence wall 108. The flow barrier material 402 may be permeable andfilter the impounded flow downstream to avoid overtopping of thebarriers 402 and fence wall 108. Silt fence material (e.g.,polypropylene) may be cut to the required dimensions and shape. It maybe pre-creased (e.g., with a hot iron) to make a two inch flap on thevertical and horizontal side of the triangular flow barrier 402. Theflow barrier 402 may be attached to the fence wall 108 and apron 102with hot glue, rivets, or other means. If hot glue is used, immediatelyafter attaching the flow barrier 402, the hot glue may be spread evenlyon the two inch flap with the help of a roller.

Referring now to FIG. 5, a perspective view of a flow barrier attachedto an apron and fence according to aspects of the present disclosure isshown. It can be seen that to further secure the barrier to the apron,plastic snap-on rivets 502 may be installed, possibly using a backingplate 504. Another embodiment uses an industrial sewing machine to sewthe barrier to the fence and apron. Selection of the attachment methodcan be based on cost and ease of operation.

Referring now to FIG. 6, an exploded view of a flow barrier, fence, andfence post attachment according to aspects of the present disclosure isshown. The posts 110 may be metal posts with stabilizers to support thefence wall 108. The spacing of the posts 110 can be fixed or allowed tovary depending on the site and local climate. In some embodiments,computer simulations may be used to determine post spacing. The fenceand the vertical flap of the flow barrier (glued area) may be attachedto the fence post using a metal backing plate 602, with plastic pushtype rivets 604 as shown in FIG. 6. The surface of the metal backingplate 602 and post 110 may be roughened to prevent pulling of the fabricfrom one post interval to another, thus preventing excessive stretching.Securing the flow barrier 402 to the fence post with roughened post,metal backing plate, and plastic rivets, will give additional strengthfor the fence material to withstand the impoundment load.

Referring now to FIG. 7 a close up view of a fence post attached to asilt fence according to aspects of the present disclosure is shown. Thesilt fence 700 of FIG. 7 is similar to those previously described butillustrates another method of attaching the fence posts 110 to the fencewall 108. No flow barrier is illustrated in FIG. 7 (for the sake ofsimplicity) but it is understood that the present method of fence postattachment may be appropriately adaptable for locations along the fencewall 108 where a flow barrier is attached to the fence post 110 see,e.g., FIG. 6).

The post 110 attaching to the fence wall 108 may be a steel post. Thepost 110 may be at least as tall as the fence wall 108 to securelyretain the fence wall 108, and be securely implantable into the ground.In the present embodiment, the fence wall 108 is sandwiched between thepost 110 and a tubular securement 702. The post 110 may be shaped toconform to the contour of the tubular securement 702 in order toincrease holding strength. This also isolates the individual fencesections such that stretching, sagging, or other deformations in onesection will not necessarily affect adjacent sections.

The tubular securement 702 may be an electrical or mechanical tubingpiece. In one embodiment, the securement 702 is sized in lengthsubstantially similarly to the height of the portion of the post 110that is above ground in order to fully secure the fence 108 to the post110. The securement 702 may attach to the post with sheet metal screws704 or other fasteners. In one embodiment, the length of the sheetmetalscrews 704 will be chosen such that the screws do not extend completelythrough the securement 702. Part or all of the fence 700 may bepreassembled before being installed, or the post(s) 110 may be insertedinto the ground and the fence wall 108 attached to the posts 110 withthe securement 702 using hand tools the site of installation.

Thus, the present invention is well adapted to carry out the objectivesand attain the ends and advantages mentioned above as well as thoseinherent therein. While presently preferred embodiments have beendescribed for purposes of this disclosure, numerous changes andmodifications will be apparent to those of ordinary skill in the art.Such changes and modifications are encompassed within the spirit of thisinvention as defined by the claims.

1. A silt fence structure comprising: a water permeable fence; an apronhaving first and second edges, the first edge providing a buriedupstream apron toe, the second edge attaching to the water permeablefence and providing a buried downstream apron toe; and at least one flowbarrier attached to the water permeable fence and to the apron so as toimpede lateral runoff flow along the water permeable fence.
 2. The siltfence structure of claim 1, wherein the water permeable fence provides aburied fence toe proximate the buried downstream apron toe.
 3. The siltfence structure of claim 1, wherein the at least one flow barrier has agenerally right triangular shape providing first and second edgesattaching to the water permeable fence and the apron, respectively. 4.The silt fence structure of claim 1, wherein the at least one flowbarrier has a curved triangular shape with first and second straightlegs attaching to the water permeable fence and the apron, respectively,and a curved leg spanning between the water permeable fence and theapron.
 5. The silt fence structure of claim 1, wherein the at least oneflow barrier comprises a plurality of flow barriers attaching to thewater permeable fence at substantially the same location.
 6. The siltfence structure of claim 1, further comprising at least one fence postattached to the water permeable fence.
 7. The silt fence structure ofclaim 6, further comprising a backing plate, the backing plate beingpositioned along the fence post and attached to the fence post such thatthe water permeable fence and the flow barrier interpose the backingplate and the fence post where the flow barrier is attached to thefence.
 8. The silt fence structure of claim 1, further comprising afloculant applied to the apron so as to disperse into runoff over theapron.
 9. The silt fence structure of claim 1, further comprising atleast one geo textile pouch containing floculant attached to the apronso as to disperse floculant into runoff over the apron.
 10. The siltfence structure of claim 1, wherein the apron is further comprised offirst and second layers with a floculant placed therebewteen so as todisperse into runoff over the apron.
 11. A silt fence structure forcontrolling erosion on a sloped ground surface having an upstreamdirection and a downstream direction, the structure comprising: a waterpermeable fence providing a fence toe buried in the ground surface; aplurality of flow barriers; an apron having first and second edges, thefirst edge providing an upstream apron toe and the second edge providinga downsteam apron toe; and a plurality of fence posts, each with a firstend buried in the ground surface and extending generally upwardtherefrom; wherein the apron is attached to the fence proximate thesecond edge of the apron and extends from the fence in the upstreamdirection, the fence is attached to the apron proximate the fence toe,and the upstream and downstream apron toes are buried in the groundsurface; wherein each of the plurality of flow barriers is attached tothe filter fence along a first edge of the flow barrier and attached tothe apron along a second edge of the flow barrier; and wherein at leastone of the plurality of fence posts attaches to the fence proximate alocation connecting to at least one of the plurality of flow barriers.12. The silt fence structure of claim 11, further comprising at leastone backing plate positioned on a side of the fence opposite at leastone of the plurality of fence posts and positioned such that at leastone of the plurality flow barriers and the fence are sandwiched betweenthe backing plate and the fence post.
 13. The silt fence structure ofclaim 11, wherein at least one of the plurality of flow barriers has agenerally right triangular shape providing first and second edgesattaching to the water permeable fence and the apron, respectively. 14.The silt fence structure of claim 11, wherein at least one of theplurality of flow barriers has a curved triangular shape with first andsecond straight legs attaching to the water permeable fence and theapron, respectively, and a curved leg spanning between the waterpermeable fence and the apron.
 15. The silt fence structure of claim 11,wherein at least two of the plurality of flow barriers attach to thewater permeable fence at substantially the same location.
 16. The siltfence structure of claim 11, further comprising a floculant applied tothe apron such that the floculant disperses into runoff on the apron.17. A method of controlling erosion on a sloped soil surface having anupstream and a downstream direction, the method comprising: planting aplurality of fence posts into the soil; attaching a silt fence betweenthe plurality of posts; providing an apron attached along the fence, theapron forming a downstream apron toe proximate the fence and extendingoverground therefrom in an upstream direction to form an upstream aprontoe; burying the downstream and upstream apron toes in the soil; andattaching at least one flow barrier between the fence and the apron. 18.The method of claim 17, further comprising spraying a floculant onto theapron.
 19. The method of claim 17, further comprising imbuing the apronwith a floculant.